The present invention relates to a scanning electron microscope (hereinafter, referred to as “SEM: Scanning Electron Microscope”) and an image detecting method thereof for detecting an image of a surface of an observation target by irradiating a convergent electron beam to the observation target such as a semiconductor wafer and detecting electrons emitted from the irradiated position, and particularly relates to a technique effectively applied to an SEM semiconductor wafer inspection apparatus required to take high-magnification images, a review SEM for observing in greater detail defects detected in the semiconductor wafer, and a length measurement SEM for measuring a pattern formed on the semiconductor wafer, etc.
As a technique that the present inventors have studied, the following techniques are conceivable in, for example, the SEM semiconductor wafer inspection apparatus, and the review SEM, the length measurement SEM, etc. (for example, see Japanese Patent Laid-Open Publication No. 2003-16983).
Along with miniaturization of the patterns on the semiconductor wafer, control of a front-end manufacture process of semiconductor is becoming more and more difficult; detection of defect, observation, and dimension measurement of pattern width in the optical microscope are becoming difficult; and inspection, review (reexamination), and length measurement are performed based on the images taken by the SEM.
The SEM emits an electron beam converged on a surface of the observation target, and detects a secondary electron or reflected electron emitted from the irradiated position. When the irradiated position of the electron beam is two-dimensionally moved, a two-dimensional image is taken.
Expected values of the secondary electron and the reflected electrons then emitted upon irradiation of the electron beam are known to be proportional to a beam current which controls an amount of electron beams to be irradiated. The number of the emitted secondary electrons is not completely the same in the case of the same beam current, a variation occurs in the emitted number, and the variation is proportional to the emitted electron number to one-half power. This variation is generally known to be a main factor of noise of the detected image in SEM.
Regarding an S/N of detected images, the symbol “S” indicating a signal is proportional to the beam current and the variation which is a noise component is proportional to the beam current to one-half power, so that the S/N is proportional to the beam current to the one-half power. It is known that, for this characteristic, in order to obtain the image with the good S/N, the image has to be detected by use of the beam current which is as large as possible.
However, it is known that, when the beam current is increased, aberration of an electro-optical system is increased and a beam diameter is increased. Therefore, there has been a problem that high-resolution images cannot be obtained. As a technique for solving this problem, a noise reduction technique which is performed by addition through a frame is known. This is a method in which a plurality of images within the same region are taken and each of the images is used as a frame and the frames are integrated to synthesize a final image. When this method is employed, the S/N is proportional to the number of added frames to the one-half power, while the image taken time increases in proportion to the number of added frames.
By the SEM having such a characteristic, defects of the semiconductor are reviewed or the dimensions of the semiconductor pattern are measured. However, when such processes are to be performed, two types of images are generally taken, i.e., low-magnification image detection and high-magnification image detection are performed. For example, in the review SEM for reviewing the defects of semiconductor, the defect is enlarged at high magnification and displayed in accordance with defect coordinates outputted by an inspection device for detecting the defects. However, accuracy of the defect coordinates outputted by the inspection device is bad within a visual field of the high-magnification image. Therefore, first, a defect position is specified by comparing the low-magnification defect image with a reference image which is a normal image having the same pattern as that of the defect image, and the defect position is enlarged to obtain the high-magnification image.
On the other hand, in the length measurement SEM, in order to determine a pattern to be measured, a low-magnification image is similarly detected. The image is taken subsequently at high magnification in order to perform measurement with high accuracy, and line width etc. is measured from the taken image.